Embryonic Hemispheres Research Articles

Extraembryonic Membranes and Placentation in the Mexican Snake Conopsis lineata.

Extraembryonic membranes provide protection, oxygen, water, and nutrients to developing embryos, and their study generates information on the origin of the terrestrial egg and the evolution of viviparity. In this research, the morphology of the extraembryonic membranes and the types of placentation in the viviparous snake Conopsis lineata are described through optical microscopy during early and late gestation. When embryos develop inside the uterus, they become surrounded by a thin eggshell membrane. In early gestation, during stages 16 and 18, the embryo is already surrounded by the amnion and the chorion, and in a small region by the chorioallantois, which is product of the contact between the chorion and the growing allantois. A trilaminar omphalopleure covers the yolk sac from the embryonic hemisphere to the level of the equator where the sinus terminalis is located, and from there a bilaminar omphalopleure extends into the abembryonic hemisphere. Thus, according to the relationship of these membranes with the uterine wall, the chorioplacenta, the choriovitelline placenta, and the chorioallantoic placenta are structured at the embryonic pole, while the omphaloplacenta is formed at the abembryonic pole. During late gestation (stages 35, 36, and 37), the uterus and allantois are highly vascularized. The allantois occupies most of the extraembryonic coelom and at the abembryonic pole, it contacts the omphaloplacenta and form the omphalallantoic placenta. This is the first description of all known placenta types in Squamata for a snake species member of the subfamily Colubrinae; where an eggshell membrane with 2.9 μm in width present throughout development is also evident. The structure of extraembryonic membranes in C. lineata is similar to that of other oviparous and viviparous squamate species. The above indicates not only homology, but also that the functional characteristics have been maintained throughout the evolution of the reproductive type.

Calcium Signaling during Cortical Apical Dendrite Initiation: A Role for Cajal-Retzius Neurons.

The apical dendrite of a cortical projection neuron (CPN) is generated from the leading process of the migrating neuron as the neuron completes migration. This transformation occurs in the cortical marginal zone (MZ), a layer that contains the Cajal-Retzius neurons and their axonal projections. Cajal-Retzius neurons (CRNs) are well known for their critical role in secreting Reelin, a glycoprotein that controls dendritogenesis and cell positioning in many regions of the developing brain. In this study, we examine the possibility that CRNs in the MZ may provide additional signals to arriving CPNs, that may promote the maturation of CPNs and thus shape the development of the cortex. We use whole embryonic hemisphere explants and multiphoton microscopy to confirm that CRNs display intracellular calcium transients of <1-min duration and high amplitude during early corticogenesis. In contrast, developing CPNs do not show high-amplitude calcium transients, but instead show a steady increase in intracellular calcium that begins at the time of dendritic initiation, when the leading process of the migrating CPN is encountering the MZ. The possible existence of CRN to CPN communication was revealed by the application of veratridine, a sodium channel activator, which has been shown to preferentially stimulate more mature cells in the MZ at an early developmental time. Surprisingly, veratridine application also triggers large calcium transients in CPNs, which can be partially blocked by a cocktail of antagonists that block glutamate and glycine receptor activation. These findings outline a model in which CRN spontaneous activity triggers the release of glutamate and glycine, neurotransmitters that can trigger intracellular calcium elevations in CPNs. These elevations begin as CPNs initiate dendritogenesis and continue as waves in the post-migratory cells. Moreover, we show that the pharmacological blockade of glutamatergic signaling disrupts migration, while forced expression of a bacterial voltage-gated calcium channel (CavMr) in the migrating neurons promotes dendritic growth and migration arrest. The identification of CRN to CPN signaling during early development provides insight into the observation that many autism-linked genes encode synaptic proteins that, paradoxically, are expressed in the developing cortex well before the appearance of synapses and the establishment of functional circuits.

Placentation in the Mexican scincid lizard Plestiodon brevirostris (Squamata: Scincidae).

Viviparity is the reproductive pattern in which females gestate eggs within their reproductive tract to complete their development and give birth to live offspring. Within extant sauropsids, only the Squamata (e.g., snakes, lizards, and amphisbaenians) evolved viviparity, representing 20% of the existing species. The genus Plestiodon is represented by 43 species and is one of the most widely distributed genera of the Scincidae in Mexico. The goal of this research has been to determine the placental morphology and ontogeny during gestation in the lizard Plestiodon brevirostris. Specimens were dissected to obtain the embryonic chambers and the embryos were categorized to carry out the correlation between the development stage and the placenta development. The embryonic chambers were processed using the conventional histological technique for light microscopy. The identified embryonic stages were 4, 29, 34, 36, and 39. A thin eggshell surrounds the egg in early developmental stages; however, this structure is already absent in the embryonic hemisphere during the developmental stage 29. The results revealed that P. brevirostris is a lecithotrophic species, but a close maternal-fetal relationship is established by tissue apposition. Ontogenically, the placental types that form in the embryonic hemisphere are the chorioplacenta, choriovitelline placenta, and chorioallantoic placenta; whereas the omphaloplacenta is formed in the abembryonic hemisphere. The structure of the chorioallantoic placenta in P. brevirostris suggests that it may play a role during gas exchange between the mother and the embryo, due to the characteristics of the epithelia that comprise it. The structure of embryonic and maternal epithelia of the omphaloplacenta suggests a role in the absorption of the eggshell during gestation and possibly in the transport or diffusion of some nutrients. In general, it is evident that ontogeny and placental characteristics of P. brevirostris match those of other species of viviparous lecithotrophic scincids.

Novel placental structure in the Mexican gerrhonotine lizard, Mesaspis viridiflava (Lacertilia; Anguidae).

The evolution of viviparity alters the physical relationship between mothers and offspring and the prevalence of viviparity among squamate reptiles presents an opportunity to uncover patterns in the evolution of placental structure. Understanding the breadth of this diversity is limited because studies of placental structure and function have emphasized a limited number of lineages. We studied placental ontogeny using light microscopy for an embryological series of the Mexican gerrhonotine lizard, Mesaspis viridiflava. This species develops an elaborate yolk sac placenta, an omphaloplacenta, which receives vascular support arising in a structure known only from other gerrhonotine lizards. A prominent feature of the omphaloplacenta is a zone of uterine and embryonic epithelial cell hyperplasia located at the upper shoulder of the yolk mass, often extending above the yolk mass. The omphaloplacenta covers more than one-half of the surface area of maternal-embryonic contact. The chorioallantoic placenta has a more restricted distribution because the allantois remains in the embryonic hemisphere of the egg throughout development and lies internal to the vascular support for the omphaloplacenta in areas where they overlap. The structural profile of the chorioallantoic placenta indicates a potential for respiratory exchange and/or hemotrophic nutritive transport, while that of the omphaloplacenta suggests that nutritive transfer is primarily via histotrophy. An eggshell is present in the earliest embryonic stages examined but regresses relatively early in development. Placental specializations of this species are consistent with a pattern of matrotrophic embryonic nutrition and have evolved in a unique lineage specific developmental pattern.

Chorioallantoic Placentation in Viperid Snakes

Chorioallantoic Placentation in Viperid Snakes

Development of yolk sac and chorioallantoic membranes in the Lord Howe Island skink, Oligosoma lichenigerum

Development of the yolk sac of squamate reptiles (lizards and snakes) differs from other amniote lineages in the pattern of growth of extraembryonic mesoderm, which produces a cavity, the yolk cleft, within the yolk. The structure of the yolk cleft and the accompanying isolated yolk mass influence development of the allantois and chorioallantoic membrane. The yolk cleft of viviparous species of the Eugongylus group of scincid lizards is the foundation for an elaborate yolk sac placenta; development of the yolk cleft of oviparous species has not been studied. We used light microscopy to describe the yolk sac and chorioallantoic membrane in a developmental series of an oviparous member of this species group, Oligosoma lichenigerum. Topology of the extraembryonic membranes of late stage embryos differs from viviparous species as a result of differences in development of the yolk sac. The chorioallantoic membrane encircles the egg of O. lichenigerum but is confined to the embryonic hemisphere of the egg in viviparous species. Early development of the yolk cleft is similar for both modes of parity, but in contrast to viviparous species, the yolk cleft of O. lichenigerum is transformed into a tube-like structure, which fills with cells. The yolk cleft originates as extraembryonic mesoderm is diverted from the periphery of the egg into the yolk sac cavity. As a result, a bilaminar omphalopleure persists over the abembryonic surface of the yolk. The bilaminar omphalopleure is ultimately displaced by intrusion of allantoic mesoderm between ectodermal and endodermal layers. The resulting chorioallantoic membrane has a similar structure but different developmental history to the chorioallantoic membrane of the embryonic hemisphere of the egg.

Uterine and chorioallantoic angiogenesis and changes in the uterine epithelium during gestation in the viviparous lizard, niveoscincus conventryi (squamata: scincidae)

We used immunofluorescent confocal microscopy and scanning electron microscopy to quantify uterine vascularity and to describe uterine surface morphology during gestation in pregnant females of the lecithotrophic lizard Niveoscincus coventryi. As uterine angiogenesis and epithelial cell morphology are thought to be under progesterone control, we studied the effect of a progesterone receptor antagonist (mifepristone) on uterine and chorioallantoic microvasculature and features of the uterine epithelial surfaces. Although intussuceptive angiogenesis was observed in both, uterine and chorioallantoic, vascular beds during gestation, the only significant increases were in the diameters of the uterine vessels. An ellipsoid vessel-dense area grows in the mesometrial hemisphere of the developing conceptus, which parallels the expansion of the allantois to form the chorioallantoic placenta. Uterine surface topography changed during gestation. In particular, uterine blood vessels bulge over the luminal surface to form marked ridges on the uterine embryonic hemisphere, especially during the last stage of pregnancy, and ciliated cells are maintained in the embryonic and abembryonic hemispheres but disappear in both the mesometrial and antimesometrial poles. This distinct regionalization of uterine ridges and ciliated cells in the uterine surface and in the shape of the epithelial component of the chorion might be related to the function of both chorioallantoic and yolk sac placentae during gestation. There was no significant difference between females treated with or without mifepristone, which may be related to the partial function of mifepristone as a progestin antagonist and/or with the function and time of action of progesterone in the uterus during gestation in N. coventryi. Differences in the pattern of angiogenesis and uterine surface morphology during gestation among squamates may be related to the functional diversity of the uterine component of the different placentae and probably reflect its diverse evolutionary history.

Uterine and Placental Angiogenesis in the Australian Skinks, Ctenotus taeniolatus, and Saiphos equalis

The evolution of viviparity requires modifications to multiple integrated physiological features to support embryonic development during pregnancy. Embryonic growth during pregnancy is dependent upon the capacity of the uterine vascular system to satisfy increasing embryonic oxygen demand throughout gestation. We tested the hypothesis that total surface area of uterine blood vessels increases in concert with embryonic growth, and hence its oxygen demand, during gestation. We used immunofluorescence and laser-scanning confocal microscopy to quantify uterine microvascular density and morphology during gestation in the oviparous skink Ctenotus taeniolatus and in Saiphos equalis, a skink species with prolonged egg retention. For C. taeniolatus, vessel density (Nv) and vessel length-density (Lv) in the embryonic hemisphere of the uterus is 23% and 17% less, respectively, than that of S. equalis and vascular surface-area does not differ as a function of embryo stage. For S. equalis, overall Nv, Lv, and vessel diameter (Dv), does not change during the first half of gestation but increases by 36% (Nv), 44% (Lv), and 60% (Dv) by near-term embryo stages late in gestation. The chorioallantoic membrane of S. equalis increases in absolute size but vascular density does not differ as a function of embryo stage. The marked increase in uterine vascular density during late gestation coincides with the phase of rapid growth in embryo mass and concomitant increase in metabolic rate. Expansion of the uterine vascular bed in concert with embryo size and metabolism is likely to be an important transitional step in the evolution of viviparity.

An avian model for the reversal of neurobehavioral teratogenicity with neural stem cells

A fast and simple model which uses lower animals on the evolutionary scale is beneficial for developing procedures for the reversal of neurobehavioral teratogenicity with neural stem cells. Here, we established a procedure for the derivation of chick neural stem cells, establishing embryonic day (E) 10 as optimal for progression to neuronal phenotypes. Cells were obtained from the embryonic cerebral hemispheres and incubated for 5–7days in enriched medium containing epidermal growth factor (EGF) and basic fibroblast growth factor (FGF2) according to a procedure originally developed for mice. A small percentage of the cells survived, proliferated and formed nestin-positive neurospheres. After removal of the growth factors to allow differentiation (5days), 74% of the cells differentiated into all major lineages of the nervous system, including neurons (Beta III tubulin-positive, 54% of the total number of differentiated cells), astrocytes (GFAP-positive, 26%), and oligodendrocytes (O4-positive, 20%). These findings demonstrate that the cells were indeed neural stem cells. Next, the cells were transplanted in two allograft chick models; (1) direct cerebral transplantation to 24-h-old chicks, followed by post-transplantation cell tracking at 24h, 6days and 14days, and (2) intravenous transplantation to chick embryos on E13, followed by cell tracking on E19. With both methods, transplanted cells were found in the brain. The chick embryo provides a convenient, precisely-timed and unlimited supply of neural progenitors for therapy by transplantation, as well as constituting a fast and simple model in which to evaluate the ability of neural stem cell transplantation to repair neural damage, steps that are critical for progress toward therapeutic applications.

A new monoclonal antibody, A3B10, specific for astrocyte‐lineage cells recognizes calmodulin‐regulated spectrin‐associated protein 1 (Camsap1)

Recent studies of adult neurogenesis of the mammalian central nervous system have suggested unexpected plasticity and complexity of neural cell ontogenesis. Redefinition and reconstitution of cell classification and lineage relationships, especially between glial and neural precursors, are an urgent and crucial concern. In the present study, we describe a new monoclonal antibody, A3B10, which was produced by immunizing mice with the membrane fraction prepared from astrocyte-enriched primary neural cell cultures. Immunohistochemistry of brain sections, including brains from glial fibrillary acidic protein (GFAP)-deficient mice and primary mixed neural cell cultures, as well as immunoblot analysis and immunoelectron microscopy, have revealed that 1) A3B10 recognizes a majority of cells in ependyma in neonatal and adult rats, 2) A3B10 stains almost all GFAP(+) cells and some S100beta(+) cells in the corpus callosum, 3) A3B10 specifically stains astrocytes in vitro in primary cultures of rat embryonic cerebral hemispheres, 4) A3B10 equally stains ependymal cells of wild-type and GFAP-deficient mice, and 5) A3B10 antigen might construct intermediate filament bundles with GFAP and/or vimentin. These data suggested that the antibody labels a wide array of astorcytic-lineage cells including astrocytes, astrocyte precursors, and neural stem cells. Screening a cDNA library derived from rat embryonic brain has revealed that the antibody recognizes calmodulin-regulated spectrin-associated protein 1 (Camsap1). Thus this antibody may provide not only a new marker to identify astrocyte-lineage cells but also a new target molecule to elucidate the ontogeny, development, and pathophysiological functions of astrocyte-lineage cells.

Morphological Variation in the Allantoplacenta Within the Genus Mabuya (Squamata: Scincidae)

The type IV allantoplacenta has been described for the New World tropical scincids lizards of the genus Mabuya; it possesses the greatest morphological complexity known among viviparous squamates. Although a common morphological pattern has been observed in the few species of this lineage in which the allantoplacental morphology has been studied, some morphological variations may be present among species and populations. Here, we report morphological variation of the allantoplacenta of twelve populations of the genus Mabuya distributed in different geographical areas in northern South America using light microscopy. It is found that all the populations/species conserve a general arrangement of the placental structures. In the embryonic hemisphere there are a placentome, paraplacentome, and chorionic areolas; these structures are related to histotrophic nutrition. At the abembryonic hemisphere, there are absorptive plaques for histotrophic transfer and respiratory segments for gas exchange. However, in some populations some distinctive features in the placentome were found. The presence in the uterine syncytium of non syncytialized columnar cell groups, and invasive cells and apical projections of the chorionic cells directed toward the uterine syncytium, constitute a localized endotheliochorial placenta. Likewise, variations found in the abembryonic region include a greater morphological complexity, such as the folded and delimited absorptive plaques, and highly folded regions at the abembryonic pole (folded respiratory segments integrated with folded absorptive plaques). These specializations allow a larger surface for the passage of nutrients and respiratory exchange. Replication and the regionalized differentiation of the absorptive plaques were probably instrumental in the emergence of specialized structures for nutrient transport such as the placentome and the different types of absorptive plaques. These developmental processes appear to underlie the evolution of the placental complexity within thegenus Mabuya by the morphological variation of serial homologous structures.

Uptake of dextran-FITC by epithelial cells of the chorioallantoic placentome and the omphalopleure of the placentotrophic lizard,Pseudemoia entrecasteauxii

Placental nutrient provision has evolved in multiple lineages of squamate reptiles and although possible structural specializations for placentotrophy have been described in a variety of species, neither the pathways nor the mechanisms of placental transfer are known. Lizards of the Australian genus Pseudemoia are placentotrophic and have elaborate placental structures that are thought to enhance nutrient transfer. The chorioallantoic placenta, which occupies the embryonic hemisphere of the egg, is regionally diversified into a large area with low epithelial height and a smaller placentome with cuboidal or columnar epithelia. Both regions are underlain by an extensive vascular bed. The abembryonic hemisphere of the egg is covered by an omphaloplacenta, which is similar to the placentome in having cuboidal or columnar epithelia but with a different embryonic vascular supply. We tested the hypothesis that embryonic epithelial cells of the placentome and the omphaloplacenta of Pseudemoia entrecasteauxii are each capable of endocytosis. Embryos (stages 33-39) with intact extraembryonic membranes were surgically removed from the uterus and incubated in a solution containing fluorescein isothiocyanate-dextran (77,000 MW). The fluorescent label was detected in the cytoplasm of scattered populations of epithelial cells in both placental regions of all embryonic stages. We conclude that both the placentome and the omphaloplacenta of P. entrecasteauxii are sites of histotrophic nutrient transport. However, there are histological and cytological differences in the embryonic epithelia of these two placental regions. The histological differences reflect differences in the evolutionary precursors of each tissue. The cytological differences likely portray different functional specializations.

Allantoplacental ultrastructure of an Andean population ofMabuya (Squamata, Scincidae)

Mabuya species are highly matrotrophic viviparous lizards with Type IV epitheliochorial allantoplacenta. The allantoplacenta of an Andean population of this genus, currently assigned to Mabuya sp., possesses specializations related to histotrophic nutrition at the embryonic hemisphere (placentome, paraplacentome, and chorionic areolas), while at the abembryonic hemisphere it has a mixed function: histotrophic transfer (absorptive plaques) and hemotrophic nutrition (gas exchange in respiratory segments). These placental specializations were studied using high-resolution light microscopy and transmission electron microscopy, and were compared with those found in other squamate reptiles and eutherian mammals. Cytological features of the placentome suggest that this is an important region for nutritional provision; the paraplacentome also shows characteristics for nutrient transfer, especially lipids. Chorionic areolas allow the absorption of glandular products, as well as uterine and chorionic cellular debris produced by lysis of some cells of both epithelia during areola formation. In the absorptive plaques both uterine and chorionic epithelia are firmly attached and their cellular apices exhibit electron-dense granules that could be related to autocrine and paracrine functions. The short interhemal distance found in the respiratory segments confirms their role in gas exchange. A common feature of all regional specializations in the Mabuya sp. allantoplacenta is the presence of lipids in the interacting chorionic and uterine epithelia, suggesting that lipids are transferred throughout the entire embryonic chamber; placental transfer of lipids may be the principal fetal energy and lipid source in this species. In spite of this feature, each one of the specialized areas of the allantoplacenta has different features suggesting particular functions in the transfer of nutrients (as ions, lipids, proteins, amino acids, sugar, water, and gases), and in the possible synthesis of hormones and proteins. The placental complexity observed in this species of Mabuya is greater than in any other reptile, and resembles that of eutherian mammals: Each one of these specializations of the placental membranes in Mabuya sp. is similar to those found among different eutherian mammals, indicating a very impressive evolutionary convergence at the histological and cytological levels between both clades. However, no eutherian mammal species simultaneously displays all of these specializations in the embryonic chamber as does Mabuya sp.

Early Asymmetry of Gene Transcription in Embryonic Human Left and Right Cerebral Cortex

The human left and right cerebral hemispheres are anatomically and functionally asymmetric. To test whether human cortical asymmetry has a molecular basis, we studied gene expression levels between the left and right embryonic hemispheres using serial analysis of gene expression (SAGE). We identified and verified 27 differentially expressed genes, which suggests that human cortical asymmetry is accompanied by early, marked transcriptional asymmetries. LMO4 is consistently more highly expressed in the right perisylvian human cerebral cortex than in the left and is essential for cortical development in mice, suggesting that human left-right specialization reflects asymmetric cortical development at early stages.

Placentation in the Mexican lizard Sceloporus mucronatus (Squamata: Phrynosomatidae)

We used light microscopy to study placental structure of the lizard Sceloporus mucronatus throughout 6 months of embryonic development. Three stages of placental development could be assigned to embryos based on the arrangement of the extraembryonic membranes. A highly vascular choriovitelline placenta was present in the embryonic hemisphere and a nonvascular bilaminar omphalopleure covered most of the abembryonic hemisphere of the egg during embryonic Stages 10-28. A chorioallantoic placenta replaced the choriovitelline placenta by embryonic Stage 29 and an omphaloplacenta covered the abembryonic hemisphere at this stage. The combination of these two placental types occurred in Stage 29-36 embryos. The final stage of placentation, embryonic Stages 37-40, was characterized by an omphalallantoic placenta in the abembryonic hemisphere and a chorioallantoic placenta in the embryonic hemisphere of the egg. The choriovitelline and chorioallantoic placentae are well vascularized, with closely apposed maternal and embryonic blood vessels. These structures are the most likely sites of respiratory exchange. In contrast, the omphaloplacenta and omphalallantoic placentae contain cuboidal or columnar epithelia and these structures may function in histotrophic exchange. Placentation of S. mucronatus is similar to that of predominantly lecithotrophic species in other squamate lineages suggesting that the evolution of this placental morphology is a response to similar factors and is independent of phylogeny.

VAP1, with cystatin C motif, an oocyte protein encoded by a novel ovarian-specific gene during oogenesis in the common brushtail possum (Trichosurus vulpecula)

In the brushtail possum oocyte, vesicles accumulate in a polarized fashion at the vegetal pole and cytoplasm rich in mitochondria and containing the germinal vesicle comprise the animal pole. During cleavage to early blastocyst stages, animal pole cytoplasm locates to the cells of the embryonic hemisphere (pluriblast) and vegetal pole vesicular cytoplasm to cells of the abembryonic hemisphere (trophoblast). Previously identified 16 amino acid residues, associated with the vesicle-rich cytoplasm were used for molecular cloning and characterization of a vesicle associated protein, VAP1. The degenerate primer was used in a 3'RACE for vap1 gene cloning. The cDNA encoding VAP1 was 516 bp in length with no significant homologies and coded for 172 amino acid residues for the mature protein. The N-terminal domain of VAP1 showed a structural homology to the cysteine protease inhibitor, Cystatin. Gene expression studies during oogenesis revealed that vap1 had an ovary-specific, possibly oocyte-specific expression, which occurs during follicle formation and growth and in adult ovaries. Recombinant VAP1 fusion protein generated polyclonal antibodies in the mouse and in the brushtail possum.

Ultrastructure of the placentae of the natricine snake, Virginia striatula (Reptilia: Squamata).

Virginia striatula is a viviparous snake with a complex pattern of embryonic nutrition. Nutrients for embryonic development are provided by large, yolked eggs, supplemented by placental transfer. Placentation in this species is surprisingly elaborate for a predominantly lecithotrophic squamate reptile. The embryonic-maternal interface consists of three structurally distinct areas, an omphalallantoic placenta and a regionally diversified chorioallantoic placenta. The chorioallantoic placenta over the embryonic hemisphere (paramesometrial region) of the egg, features close apposition of embryonic and uterine blood vessels because of the attenuate form of the interceding epithelial cells. The periphery of the chorioallantoic placenta, which is adjacent to the omphalallantoic placenta, is characterized by a simple cuboidal uterine epithelium apposed to a stratified cuboidal chorionic epithelium. There are no sites with attenuate epithelial cells and close vascular apposition. The morphology of the omphalallantoic placenta is similar to that of the peripheral chorioallantoic placenta, except that the height of uterine epithelial cells is greater and allantoic blood vessels are not associated with the embryonic epithelium. The functional capabilities of the three placental regions are not known, but structural characteristics suggest that the omphalallantoic placenta and peripheral zone of the chorioallantoic placenta are sites of nutritional provision via histotrophy. The paramesometrial region of the chorioallantoic placenta is also nutritive, in addition to functioning as the primary embryonic respiratory system. The structure of the chorioallantoic placenta of V. striatula is a new placental morphotype for squamate reptiles that is not represented by a classic model for the evolution of reptilian placentation.

Influence of cell density and thyroid hormone on glial cell development in primary cultures of embryonic rat cerebral hemisphere

The influence of cell density and thyroid hormone (TH) on the development of astrocytes and oligodendrocytes was investigated in primary cultures prepared from rat cerebral hemisphere on embryonic day (E)18. At the beginning of the culture, most of the cells were microtubule-associated protein 2 (MAP2)-positive neurons, whereas O1-positive oligodendrocytes and glial fibrillary acidic protein (GFAP)-positive astrocytes were rarely observed. After the cells were maintained in serum-free defined medium, astrocytes developed at high cell density but rarely at a low one. When leukemia inhibitory factor (LIF) was supplemented in low-density cultures, the levels of GFAP expression markedly increased to almost the same extent as in high-density culture without TH. This suggests that, in low-density cultures, astrocyte progenitors could not differentiate because of insufficient astrocyte-inducing factors. Interestingly, the addition of TH increased GFAP expression levels only at high density. The number of oligodendrocytes increased with TH addition at both cell densities, although the effects were more remarkable at high density. These results suggest that cell density and TH are pivotal factors in the development of both astrocytes and oligodendrocytes. It is also suggested that the effects of TH on glial cell development could be accelerated via cell-cell communications.

Aspects of the histology of reproduction in the female Angolan free‐tailed bat Mops condylurus (Microchiroptera: Molossidae) in Mpumalanga, South Africa

AbstractThe reproductive pattern of the female Angolan free‐tailed bat Mops condylurus (A. Smith, 1933) was investigated at two localities in Mpumalanga. Female Angolan free‐tailed bats displayed a seasonal bimodally polyoestrous reproductive pattern. Parturition in December was followed closely by a post‐partum oestrus. Ovarian follicle and endometrium cycles also closely followed the breeding season. The reproductive tract displayed complete dextral dominance with the right ovary and uterus being functionally dominant. Luteal activity was maintained throughout pregnancy and healthy corpora lutea were observed until late in pregnancy. Shedding of the zona pellucida occurred in the oviduct in the region of the ampulla–isthmus junction. The embryo entered the uterus as a morula, where it was transformed into a unilaminar, free‐lying blastocyst. A unique feature of the pre‐implantation uterus was the formation of a distinct decidua at the implantation site on the mesometrial side. Implantation in the Angolan free‐tailed bat was centric and superficial. First attachment of the trophoblast to the endometrium was mesometrial. The formation of a pre‐placental pad of trophoblast cells was also initiated on the mesometrial side, while orientation of the embryonic disc was anti‐mesometrial. Placental development occurred in the abembryonic hemisphere. Proliferation of the trophoblast over the embryonic hemisphere was slow and a bilaminar arrangement of the trophoblast was not observed in this region until after the primitive streak stage.

Uterine morphology during the annual cycle in Chalcides ocellatus tiligugu (Gmelin) (Squamata: scincidae).

The uterus of the viviparous skink Chalcides ocellatus tiligugu was studied by SEM and LM during the annual cycle. Three functional phases were identified: preovulatory (spring), gestatory (summer), and quiescent (autumn-winter), characterized by changes in the uterine wall (mainly the endometrial layer). In the preovulatory phase, the uterine wall increases in thickness; its luminal epithelium has ciliated cells and two types of unciliated secretory cells. The first type secretes sulfated glycosaminoglycans (GAGs), which form the amorphous inner layer of the eggshell membrane; the second type secretes acidic glycoproteins that form the intrafibrillar matrix of the outer layer of the eggshell membrane. The lamina propria contains simple alveolar glands that secrete the collagen fibers of the eggshell membrane. During the gestatory phase, the glycoproteins produced by secretory cells of the second type have histotrophic activity for the developing embryo. The uterus widens to form incubation chambers with two hemispheres, one embryonic and the other abembryonic. Both a chorioallantoic placenta and an omphaloplacenta with histotrophic activity are present in late gestation. The chorioallantoic placenta, with aspects of a Weekes (1935) Type III placenta, develops in the embryonic hemisphere. The omphaloplacenta forms at the vegetative pole of the egg and shows cellular hypertrophy of the bilaminar omphalopleure and uterus. During the quiescent phase, the uterus gradually decreases in thickness and activity; its luminal epithelium does not show secretory activity. The annual variations in the myometrial layer involved the inner circular and the outer longitudinal muscle layers.